1. Field of the Invention
The invention relates to a turbomachine. It is applicable to any type of turbomachine for use on land or in aviation, and more particularly is applicable to airplane turbojets.
In the present description, “upstream” and “downstream” are defined relative to the normal flow direction of gas (from upstream to downstream) through the turbomachine. Furthermore, the “axis” of the turbomachine is the axis of rotation of the rotor of the turbomachine. The “axial direction” corresponds to the axial direction of the turbomachine, and a “radial direction” is any direction perpendicular to said axis. Similarly, an “axial plane” is any plane containing the axis of the turbomachine, and a “radial plane” is any plane perpendicular to said axis. Finally, unless specified to the contrary, the adjectives “inner” and “outer” are used relative to a radial direction such that an inner (i.e. radially inner) portion or face of an element is closer to the axis of the turbomachine than is an outer (i.e. radially outer) portion or face of the same element.
The invention relates to a turbomachine of the type comprising:                an annular combustion chamber;        a centrifugal compressor;        an annular diffuser serving to diffuse the gas stream leaving the compressor and to direct this gas stream towards the combustion chamber, the diffuser comprising: a radially oriented upstream portion presenting diffusion passages connected to the outlet of the compressor; a curved intermediate portion; and a downstream portion having a series of circularly spaced-apart deflector vanes; and        an outer casing externally surrounding the combustion chamber and the downstream portion.        
2. Description of the Related Art
In general, a centrifugal compressor comprises a centrifugal impeller that serves to accelerate the gas passing therethrough and thus to increase the kinetic energy of said gas.
An annular diffuser defines an annular space surrounding the outside of the compressor, downstream therefrom. The diffuser serves to slow down the speed of the gas leaving the compressor, and as a result to increase its static pressure. Diffusers may be of the vane type or of the duct type.
These two types of diffuser comprise a radially-oriented annular upstream portion that presents a series of diffusion passages connected to the outlet from the compressor in order to recover the accelerated gas leaving it. These diffusion passages are of diverging section that increases progressively going outwards so as to diffuse the flow of gas leaving the compressor. Vane type diffusers use a series of circularly spaced-apart diffusion vanes that form diffusion passages between one another. In duct type diffusers, the diffusion passages are constituted by duct or pipe elements.
Downstream from said upstream portion, diffusers have an intermediate portion that is curved so as to curve the flow path through the diffuser and direct the flow of gas towards the combustion chamber.
Downstream from this intermediate portion, diffusers have an annular downstream portion comprising a series of circularly spaced-apart deflector vanes serving to deflect the flow of gas and thus reduce the gyration of the flow of gas leaving the diffusion passages, before said flow enters into the combustion chamber.
FIGS. 1 and 2 show a known example of a turbomachine of the above-outlined type comprising, in the gas flow direction: a centrifugal compressor 110, an annular diffuser 120 having deflector vanes 123, and an annular combustion chamber 140. An outer casing 132 surrounds the outside of the combustion chamber 140 and the diffuser 120.
The diffuser 120 comprises: a radially-oriented upstream portion 121 that presents diffusion passages 122; a curved intermediate portion 124; and a downstream portion 125 having a series of circularly spaced-apart deflector vanes 126.
The flow passage is defined as being the envelope defining the space in which the gas flows, i.e. where the gas stream passes.
The flow passage through the upstream and intermediate portions 121 and 124 is defined between a first cheekplate 127 and a second cheekplate 128. The flow passage through the downstream portion 126 is defined on the inside by the first cheekplate 127 and on the outside by an outer bushing 129.
The downstream portion 125 of the diffuser is oriented parallel to the axis A of the turbomachine. In other words, in a section plane containing the axis A of the turbomachine, the mean axis M of the flow passage at the outlet from the downstream portion 125 of the diffuser is parallel to the axis A of the turbomachine. In this configuration, the main gas flow leaving the diffuser bypasses the combustion chamber 140 on the outside.
In order to ensure that the main gas flow leaving the diffuser 120 is directed towards the combustion chamber 140, thereby feeding the combustion chamber better with gas, and in an alternative that is not shown, the downstream portion 125 of the diffuser may be inclined relative to the axis A of the turbomachine so as to be oriented towards the combustion chamber 140. In other words, in a section plane containing the axis A of the turbomachine, the mean axis M of the flow passage at the outlet from the downstream portion 125 of the diffuser forms a non-zero acute angle relative to the axis A.
The drawbacks of known diffusers stem from the fact that the parts from which they are made are difficult to assemble together. In particular, when the downstream portion 125 is inclined relative to the axis A, it is particularly difficult to fasten the outer bushing 129 onto the downstream portion 125 by brazing or by crimping, since the outside surface of the downstream portion 125 is conical, so the outer bushing 129 tends to slide towards the free end of the downstream portion 126.